WASHINGTON When President Barack Obama counselled his successor Donald Trump on the global threats he should expect once he assumed office last year, a nuclear North Korea and its unpredictable leader were at the top of the list.

But while Mr Obama practised caution in handling Mr Kim Jong Un, the blustery Mr Trump succumbed to his own penchant for reality TV showmanship, culminating with his shock decision on Thursday to agree to meet North Korea's leader and become the first sitting US president to do so.

Mr Trump's move is a sharp departure from 60 years of largely arms-length US diplomacy when it comes to North Korea, not to mention his own previous bellicose rhetoric against Pyongyang.

His willingness to take that step is a reflection of his showboat style, throwing out the diplomatic playbook and putting himself in the spotlight.

"It's hard to know whether this is just his supreme confidence that he can get a deal done with his own business experience, or whether he is calculating that he wins either way," said Mr Jim Steinberg, who served as deputy secretary of state under President Bill Clinton.

Mr Obama, like his predecessors, took a far more deliberate approach, ending up relaunching Washington's relationships with Cuba and Iran. But he did not take that same step with North Korea, once warning Pyongyang that "you don't get to bang your spoon on the table and somehow you get your way".

Officials in the Obama administration said there was a reason for this.

"Our assessment was that the North Koreans weren't serious about denuclearisation and therefore sitting down to a summit just accords legitimacy... to the regime, not just the leader, without achieving any concrete national security objective for the United States," said Mr Michael McFaul, US ambassador to Russia under Mr Obama.

Mr Trump's advisers have similar beliefs, but their boss appears ready to seize North Korea's olive branch.

White House officials said that when South Korean national security adviser Chung Eui Yong relayed Mr Kim's invitation in an Oval Office meeting on Thursday, Mr Trump readily agreed.

Veterans of the Obama and other presidencies called for caution because of North Korea's history of breaking agreements.

Mr Bill Richardson, Mr Clinton's US ambassador to the UN, who has made multiple diplomatic missions to North Korea, called the Trump-Kim summit a gamble.